1. Field of the Invention
This invention relates to a process for the removal and recovery of mercury and, if desired, other heavy metals from waste water containing such heavy metals and more particularly to such a process which comprises treating such waste water, particularly waste water produced by washing waste gases from an incinerator (such waste water being hereinafter referred to as "incinerator waste water" for brevity), with a reducing agent and then subjecting the thus treated waste water to stripping treatment in the presence of ferrous ion to obtain gases containing gaseous mercury, cooling the thus obtained gases to separate the mercury therefrom and, if necessary, further treating impurities such as the other heavy metals remaining in the resulting residue to separate them therefrom.
2. Description of the Prior Art
Combustion or waste gases from incinerators and particularly from municipal refuse incinerators contain a variety of harmful substances. It is accordingly the general practice to discharge the waste gases in the air after washing thereof, however, this practice raises a problem as to treatment of the resulting washings or waste water.
For instance, when municipal refuse including, in addition to ordinary refuse, rubbers, plastics and other combustible matters is incinerated, there will frequently be produced waste gases containing, apart from carbonic acid gas and water vapor, powders of harmful metal salts as well as harmful gases such as sulfur oxides, nitrogen oxides, chlorine and hydrogen chloride. Further, mercury which is ordinarily contained in cells and fluorescent lamps will be chemically combined with the above-mentioned hydrogen chloride and/or sulfur oxides during the incineration to give mercury-reacted chloride and/or sulfur oxides, or it will be reacted with organic matters which are not fully oxidized or decomposed, during the incineration to give organomercury compounds or complexes thereof. Thus, the waste gases may further contain such mercury-reacted compounds, organomercury compounds or complexes thereof. When the waste gases from the incinerator are treated with a cleaning liquid such as water or an alkaline solution, they will be cleaned or purified, however, a difficulty will be encountered in rendering harmless the used cleaning liquid in which the harmful substances are absorbed. It is relatively easy to convert the sulfur oxides and hydrochlorides in neutralized form contained in the waste cleaning liquid into harmless materials by chemical or physical means in order to separate and recover the resulting harmless materials from the waste cleaning liquid or in order to dilute and discharge the waste cleaning liquid. However, it has been prohibited by regulations to discharge waste water containing harmful heavy metal compounds, even in very low concentrations, to seas or rivers. Accordingly, it has been a very difficult problem to separate and recover harmful heavy metals from such waste water thereby rendering the waste water harmless.
A number of processes have been hitherto proposed for fixing and separating heavy metals such as mercury from waste water containing the heavy metals, particularly mercury. However, in order to carry out these processes on an industrial scale, there remain a number of problems to solve. Thus, there have not been known satisfactory processes for the removal of heavy metals from waste water containing the same.
For instance, quantitive determination of total mercury is carried out by an analytical method comprising adding large amounts of highly concentrated sulfuric and nitric acid to a mercury-containing waste water sample, boiling the resulting mixture, adding a reducing agent to the thus boiled mixture to strip it of the mercury using air and then determining the amount of mercury so stripped by the atomic absorption spectroscopy. However, it is difficult from the viewpoint of corrosion resistance and cost of apparatus to make use of this method for treatment of a variety of waste waters on an industrial scale.
In known literature, there is the statement that mercury compounds are susceptible to reduction with reducing agents. However, it is not particularly described to what extent or at what level the mercury compounds are reduced to strip the mercury therefrom. Moreover, it is not stated at all that such a high recovery percentage as described hereinafter is attained and waste water can be treated to such an extent that the residual mercury concentration is as low as 0.005 mg/l or less.
So far as mere removal of mercury from waste water is concerned, several techniques of treating waste water are known including a sulfide precipitation and separation technique, an alkali precipitation and separation technique and the like. In these techniques, mercury is not recovered and is discharged into the natural world in the form of a sludge, thus polluting the natural world and increasing environmental burden. In addition, the treated water has a mercury concentration as high as 0.1-0.01 mg/l and it is difficult to lower the mercury concentration to a level below 0.005 mg/l prescribed in the legal requirements in Japan. Especially, it is very difficult to apply the above-mentioned processes to the treatment of waste water which is obtained by washing waste gases from municipal refuse incinerators. For instance, the former process in which the mercury in the waste water is fixed with hydrogen sulfide as mercury sulfide and then separated from the waste water, and the latter process in which the mercury in the waste water is treated with an alkali for precipitation and separation as mercury hydroxide (which is readily converted into oxide). These processes are both suitably applied only to a case where mercury is present as ions, however, non-ionized organic mercury compounds or complexes remain as they are without causing reactions, if any in the waste water. Moreover, the sulfides or oxides tend to form fine particles and it is very difficult to precipitate and separate them from a concentrated salt solution having a concentration of about 10-15% because of a small difference in specific gravity between the particles and salt solution. This leads to a tendency that the fine particles are carried over from a separator, with the result that they remain in the waste water and thus cannot be removed. Accordingly, these processes are not suitable for economical treatment in which the waste water is concentrated to have a high concentration of mercury salts and reduce the amount of the waste water being treated. It has been confirmed that when the precipitate obtained by these processes is separated from the treated water and discharged into the natural world, the mercury compounds contained therein will be converted into harmful methylmercury compounds by bacteria in the soil. Thus the precipitate cannot be discharged in this form.
As is apparent from the above, heavy metals and particularly mercury cannot completely be removed by these known processes. Even when the mercury components are separated and discharged into the natural world, they will cause environmental pollution because of conversion into harmful substances such as methylmercury compounds. Thus, these processes are not advantageous.
In the treatment of mercury which apparently has an adverse influence on human bodies, there should not be used a process comprising discharging mercury into the natural world basically accompanied by pollution thereof but a process should be used comprising recovering the mercury from the waste water for reutilization.
The present invention is accomplished on the basis of the above standpoint and has for its object the provision of a process which comprises effectively removing and recovering mercury and, if desired, other heavy metals from incinerator waste water containing such metals thereby rendering it harmless.
The present inventors have already found that when NO.sub.3.sup.- is caused to be present in waste water obtained by washing waste gases from an incinerator, concentrations of heavy metals in the waste water are increased, and the waste water is held at high temperatures and suitable neutral to alkaline pH values, the velocity of a spinel-forming reaction can be increased with an attendant remarkable reduction of treating time. It was considered that these conditions could be conveniently applied to the washing treatment of waste gases from incinerators and it was confirmed that the above treatment could effectively be carried out on an industrial manner. We thus filed a Japanese Patent Application No. 157794/81 (Japanese Pat. Appln. Laid-Open No. 174190/82), entitled "PROCESS FOR THE TREATMENT OF WASTE WATER OBTAINED BY WASHING WASTE GASES FROM INCINERATORS". The subject matter of the thus filed application resides in a process for the treatment of waste water obtained by washing waste gases from an incinerator, which comprises the steps of charging waste gases from an incinerator into a scrubber to which water or an alkaline aqueous solution is fed as the cleaning liquid thereby washing the waste gases therewith, contacting part or all of the cleaning liquid with the high temperature waste gases prior to the introduction thereof into the scrubber, adding 0.002 mol/l to 1.0 mol/l of NO.sub.3.sup.- and 0.01 to 2.0 mols/l of Fe.sup.++ to the cleaning liquid after the gas-liquid contact, neutralizing the thus added cleaning liquid with caustic alkali, subjecting the thus neutralized liquid to oxidation treatment while keeping the pH at 6.5 to 11.5 and the temperature at 50.degree. C. or higher thereby producing a spinel type crystal compound (hereinafter called "spinel compound"), and separating the product from the neutralized cleaning liquid.
During the course of carrying out the above process, we have found that when air is blown into the used cleaning liquid, that is a waste liquid, for the oxidation reaction, most of the mercury contained in the cleaning liquid is entrained with the air in the form of vapor. Extensive studies were made on a number of reducing and oxidizing agents to find suitable time of adding the agents, amount of the agents and reaction conditions for facilitation of the entrainment. As a result, there was found a process of positively recovering mercury and, if necessary, the other heavy metals from waste water and rendering the waste water harmless. The present invention is accomplished on the basis of the above finding.